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Java Java Data Structures - Retired Exploring the Java Collection Framework Sets

Lucas Santos
Lucas Santos
19,315 Points
Posted by Lucas Santos
Lucas Santos
19,315 Points

Question about this Quiz

So I passed this quiz but did not fully understand the code. What I did not understand is how we accessed the getAuthor method from the BlogPost class if we never instantiated that class in the Blog class to begin with. Here is my code:

package com.example;

import java.util.List;
import java.util.Set;
import java.util.TreeSet;

public class Blog {
  List<BlogPost> mPosts;

  public Blog(List<BlogPost> posts) {
    mPosts = posts;
  }

  public Set<String> getAllAuthors() {
    Set <String> authors = new TreeSet<String>();
    for (BlogPost author : mPosts) {
     authors.add(author.getAuthor()); 
    }
    return authors;
  }

  public List<BlogPost> getPosts() {
    return mPosts;
  }
}

So the part with

    for (BlogPost author : mPosts) {
     authors.add(author.getAuthor()); 
    }

How is mPosts accessing BlogPosts method of getAuthor without it instantiating the BlogPost class?

Lucas Santos
Lucas Santos
19,315 Points
Lucas Santos
Lucas Santos
19,315 Points

I see, ok got it thanks a ton Chris.

1 Answer

Chris Freeman
MOD
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

The BlogPost class is accessible because it is part of the same com.example package. BlogPost is Instantiated in the Blog constructor when it assigns the lists of BlogPosts (via posts) to the member field `mPosts.

The code snippet is a bit confusing by using the parameter name "author". It would read better if it was called "post":

    for (BlogPost post : mPosts) {
     authors.add(post.getAuthor()); 
    }

Now it reads "for each 'post' in the mPosts list of BlogPosts, get the author and add it to Set of author names strings.

Lucas Santos
Lucas Santos
19,315 Points
Lucas Santos
Lucas Santos
19,315 Points

Still don't really understand, do you mean it gets instantiated when we create a new Blog object and pass in a List of BlogPosts there for giving mPosts a value?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

Correct. The challenge instructions say "[Blog] is initialized with a list of BlogPost objects." which implies there is other code somewhere that executed the equivalent of

Blog someBlog = new Blog(listOfBlogPosts);

This "list of blog posts" would then get assigned to the the mPosts in Blog. Each of the posts in the array would have the .getAuthor() method.

There is no obvious mechanism in the challenge code that shows how BlogPosts get added to the Blog list of BlogPosts other than in the Blog constructor. It would have been clearer if there were an addPost method in Blog

Tagging @Craig Dennis for comment.

Lucas Santos
Lucas Santos
19,315 Points
Lucas Santos
Lucas Santos
19,315 Points

I see so then we would do something like 

someBlog.getAllAuthors ();
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

Exactly! Additionally, other methods of Blog could reference it locally as:

// From within Blog
Set<String> allAuthors = getAllAuthors();

// From some other code
Set<String> allAuthors = someBlog.getAllAuthors();
Lucas Santos
Lucas Santos
19,315 Points
Lucas Santos
Lucas Santos
19,315 Points

I see ok think I got it but just one last concern. You said "The BlogPost class is accessible because it is part of the same com.example package."

What do you mean by that exactly? and if it wasn't in the same package we would need to instantiate it?

Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points
Chris Freeman
Chris Freeman
Treehouse Moderator 68,441 Points

What I meant by "The BlogPost class is accessible because it is part of the same com.example package" is that the package statement in Blog.java declares the Blog class to be in the package "com.example". This is the same package declared in BlogPost.java for the BlogPost class. By being in the same package, the public declarations in both java files are visible to each other. If you remove the package statement from either java file, references to BlogPost inside of Blog.java would not be resolvable.

In other words, When compiling a java file, all of the type definitions must be in either

  • the same file
  • a file in the same package
  • a file listed in an import statement from another package
  • or built-in types